Gas sampler

ABSTRACT

Apparatus for sampling gaseous exhaust emissions, on a constant flow basis. A centrifugal type extractor pump is used to draw gas through a critical flow venturi of predetermined flow rate. Exhaust emissions are diluted with air and the critical flow venturi acts as a constant volume flow device serving a metering function. Samples are taken and collected continuously during the test run, and are available to be analyzed for contaminants such for example as hydrocarbon, carbon monoxide and oxides of nitrogen.

United States Patent Kaufman 1 Oct. 24, 1972 [54] GAS SAMPLER 3,603,1559/1971 Morris ..73/42l.5

[72] Inventor: Warren F. Kaufman, Santa Ana,

Calif Przmary Examiner-Douglas Hart Attorney-Robert D. Sanborn et al.[73] Ass1gnee: Philco-Ford Corporation; Philadelphla, Pa. [57] ABSTRACT[22] Filed: March 1972 Apparatus for sampling gaseous exhaust emissions,on 2 App] 233,0 2 a constant flow basis. A centrifugal type extractorpump is used to draw gas through a critical flow venturi ofpredetermined flow rate. Exhaust emissions are Jll diluted with air andthe Critical flow venturi acts as a a n s t l s s t t s s s s Q u t t QI v t t n a [58] Field of Search ..73/42l.5, 23, 27, 60/276 tion.Samples are taken and collected continuously during the test run, andare available to be analyzed [56] References cued for contaminants suchfor example as hydrocarbon,

UNITED STATES PATENTS carbon monoxide and oxides of nitrogen.

3,593,023 7/ 197.1 Dodson ..73/42l.5 10 Claims, 6 Drawing Figures 34 I.L J

I PATENTEDom 24 m2 SHEET 1 BF 3 :x/uwr 70 Amanda?! l 1 ma M a L n M M tAwv M0 Z A r m UMIII' m GAS SAMPLER BACKGROUND OF THE INVENTION certaincontaminants. For example the emissions may not exceed: hydrocarbons 3.4grams per vehicle mile; carbon monoxide 39.0 grams per vehicle mile; andoxides of nitrogen 3.0 grams per vehicle mile.

The presence of such standards has made it imperative that the exhaustemissions from vehicle engines be tested and analyzed to determine therelative amounts of impurities therein. Much effort has gone into thedevelopment of equipment for use in this field of gas sampling and it isnow known to pass gases from an internal combustion engine through testapparatus at an accurately controlled flow rate for purposes ofdetermination, and analysis, of the relative amounts of impurities.Since the engine is breathing ambient air, and since such air is itselfa source of contaminants, care is taken that the analysis accuratelyreflects the contaminants introduced by the engine only. The apparatuscurrently available for sampling provides for introduction of diluentair and combining of the same with the exhaust gases. A proportionalpart of the diluted exhaust emissions is collected continuously, forsubsequent analysis, using a constant volume (although variabledilution) sampler. A system which satisfies these requirements, as setout by the Environmental Protection Agency, is shown in diagrammaticform and described in the identified portion of the Federal Register.(See also FIG. 1 hereof.)

The apparatus as illustrated and described utilizes a positivedisplacement pump, specifically a Roots blower, to provide the constantvolume flow which is a prerequisite to successful operation of theequipment. The volumetric flow rate is determined as a function of thenumber of revolutions of the positive displacement pump.

Apparatus of this kind is subject to a number of difficulties anddisadvantages among which are the following. Such equipment customarilyoperates in what can be termed a hostile environment. That is, theexhaust gases contain corrosive substances as well as particulatematter. Such matter tends to build up within the constant displacementpump, gradually diminishing the fine working clearances and causingvariation in displacement, and even pump failure. Because of theseproblems periodic shutdowns of the apparatus are necessary formaintenance and cleaning purposes. In addition it has not been possiblecompletely to eliminate leakage across the pump, which is a function ofgas temperature and differential pressure. The leakage factor must betaken into account in conducting tests with the equipment knownheretofore, and maintenance of this factor within predetermined limitsrequires relatively frequent calibration tests of the positivedisplacement pump, in order to insure continuing accuracy of flowmeasurement. Further, it has been found that when such pumps are used inthe mentioned hostile environment pump damage results from the presenceof excessive heat as well as the corrosive elements referred to above.

There is growing emphasis on turbine type engines and these engines mustalso be subjected to exhaust testing. As can readily be appreciated, aconstant displacement pump system of the kind shown and described in theFederal Register would have to be of enormous proportions in order todeal with the output of such engines, particularly when it is recognizedthat the pump also must handle the dilution air admitted into thesystem.

SUMMARY OF THE INVENTION I have discovered that all of the foregoingdifficulties and limitations can be eliminated in relatively simplemanner by utilizing a different constant flow device in place of thepositive displacement pump used heretofore. For this purpose I use aventuri device in combination with a pump of capacity sufficient toinsure that the venturi operates at conditions of critical flow, thatis, under conditions such that gases reach sonic speed in thethroat ofthe venturi. It will be recognized by those skilled in the art that themaximum gas coreelary it has been recognized that the gas, by

weight, reaches its maximum flow rate when passing through therestriction under the sonic, or critical, flow condition.

Since a venturi device operated in this way is inherently a constantvolume flow device, I subject the system to sufficient pressuredifference across the venturi to insure that the sonic condition isreached and maintained throughout the test. For this purpose I prefer toemploy a simple centrifugal air pump, or blower, coupled to theexpansion side of the venturi and of capacity sufficient to maintaincritical flow of the mixture of exhaust gases and diluent air throughthe venturi. If desired it is possible, in accordance with theprinciples of my invention, to use a plurality of constant flow venturigas samplers in parallel, each fed from a separate engine and preferablywith a single centrifugal pump or blower.

With the foregoing in mind it is the general objective of this inventionto provide improved sampling apparatus which will operate in apredetermined range of flow and volume in an entirely reliable andpredictable manner, and with minimal deterioration of criticalcomponents and virtual elimination of problems due to particulateaccumulation. More specifically, my improved apparatus operates withsuch accuracy of flow measurement that deviations of only 1 percent inflow rate are currently achieved, and this with a reduction of audiblenoise output and greatly prolonged life of the equipment. It is possibleto scale flow rates up or down relatively easily, and I have built unitswhich are rated at 100, 300 and 350 CFM (standard air).

The periodic calibration tests presently required on prior art systemsare not necessary with my apparatus and, in one embodiment, temperaturechanges are no longer a limitation. Additionally, variations in leakagerate are no longer of consequence. A very important advantage of theimproved system is that it is readily usable in the sampling of exhaustemissions from turbine engines.

In considering the nature and importance of the present improvement, itis desirable to record that while the critical flow characteristics ofventuris have long been recognized, it has not previously been known toutilize this characteristic to achieve the advantages mentioned above.This fact is particularly significant when it is borne in mind that inrecent years great effort and expense has been devoted to the productionof emission sampling equipment, and yet those engaged in these effortshave not advanced the art beyond the system described in the FederalRegister.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic illustrationof the prior art exhaust gas sampling system illustrated and described 1in the Federal Register;

FIG. 2 is a perspective view of the actual hardware comprising asampling system constructed in accordance with the present invention;

FIG. 3 is a diagrammatic view of the'apparatus illustrated in FIG. 2;

FIG. 4 is a sectional view, on a larger scale, illustrating a criticalflow venturi of the type used in the apparatus illustrated in FIGS. 2and 3;

FIG. 5 is a diagrammatic view of a modified'form of the inventionemploying a plurality of constant flow of venturi devices arranged inparallel; and

FIG. 6 is a perspective illustration of modified apparatus in which aheat exchanger is not employed, and which apparatus is particularlysuited for testing gas emissions from turbine engines.

DESCRIPTION OF THE PREFERRED EMBODIMENT during the entire test. It willbe understood that when a vehicle engine is under load, a larger portionof the mixture will be exhaust gas. The approved equipment is sodesigned that only a very small portion of the mixture is comprised ofcontaminants. Small samples of the mixture are drawn off and diverted toanalyzing equipment (not shown). In such apparatus, as long as flowthrough the equipment is maintained constant, the proportion of gasdrawn off will always be uniform. Since all of the contaminants (aftersuitable correction for the atmospheric conditions prevailing on the dayof the test; see probe 18 and bag 14) can be assumed to come from thevehicle exhaust, sampling of a constant fraction will permit calculationof the grams of contaminant included in the entire stream of exhaust. Asnoted above, a Roots blower has been used as the constant volumedisplacement device, and counting the number Reference is first made toFIG. 1, which illustrates present invention has elements in common withthis system. However, as will now be understood, it incorporates acritical flow venturi and associated centrifugal blower in place of thepositive displacement pump appearing in FIG. 1.

In operation of the known apparatus shown in FIG. 1 the total volumetricflow of exhaust gas, derived (throughout a simulated driving cycle) fromtail pipe adaptor l0, and from air introduced through the dilution inlet11, is metered by passing through a positive displacement pumpidentified with that term and bearing the reference numeral 12. Samplesof the exhaust gas and diluent air flow through suitable conduits andare collected in the bags or receptacles shown, respectively, at 13 and14. Filters are introduced in the conduits where required, see forexample the three-part filter assembly 15 disposed in air inlet 11, andalso the filters appearing at 16 and 17 in the gas sampling lines whichare fed by probes 18 and 19. Three-way solenoid valves 20 and 21 directsample streams either to their respective bags 14 or 13 or to discard.Leak-tight fittings and suitable shut-off devices are also employed, asare counter devices 22 and 23 which count the revolutions of theconstant displacement pump while tests are in progress and samples arebeing collected.

In approved apparatus, incorporating as it does a constant displacementpump, mixing of the entire stream of gas from an internal combustionengine with a quantity of diluted air, insures that a constant volume ofthe mixture will be translated through the equipment of revolutions ofthe pump or blower provides a direct indication-of the quantity ofmixture displaced per unit of time. It is this blower which has thelimitations, and

presents the difficulties, which my invention overcomes.

In a system of the kind shown in FIG. 1 the inlet gas temperature to thepositive displacement pump 12 must be substantially constant, in orderthat the pump may serve as a constant volume metering device. A heatexchanger H is illustrated in FIG. 1, and this heat exchanger, as isapparent from the legends appearing on the drawing includes means forboth heating and cooling the gases. This exchanger, which need not bedescribed in further detail herein, meets the requirements of constantvolume flow.

During an exhaust emission test, samples are collected in thereceptacles, or bags, 13 and 14, and analytical tests are made todetermine the hydrocarbon, carbon monoxide and oxides of nitrogen massemissions. In making such determinations use is made of suitableanalytical apparatus which is also shown diagrammatically and describedin the Regulations. Such analytical equipment need not be illustrated orconsidered herein, since, per se, it forms no part of the presentinvention.

Under the present standards a sample-collecting test is conducted whilesimulating an average trip (e.g. 7.5 miles) in an urban area, assuming acold start. A proportionate part of the diluted gas emissions iscollected continuously for analysis, such proportionate part beingdiluted with air to a constant volume.

For further and more detailed description of the system of FIG. 1, andof the manner in which the apparatus thereof is controlled andenergized, reference should be had to the Federal Register whichincludes a complete description of this system, including the apparatus(not shown) for conducting gas analyses.

Apparatus of this kind, which as noted constitutes the present standard,is subject to the difiiculties and disadvantages set forth above. Inessence the present invention eliminates thesedifficulties by separatingthe air metering function from the air pumping function. This is done byutilizing a critical flow venturi, for the metering function, incombination with a simple and compact centrifugal blower. Apparatus ofthis improved type is shown in FIGS. 2 and 3 now to be described. Laterin this specification reference will be made to FIG. 5, in which aplurality, for example two,

venturis are used in place of the single venturi device of the apparatusshown in FIG. 3.

Referring first to FIG. 2, there is illustrated apparatus which hasyielded very good results. This apparatus is shown housed in a cabinet24 and comprises an inlet conduit 25 adapted for connection to theexhaust pipe of an internal combustion engine (not shown). Environmentalair flows down through the stack 28 into a chamber 27, and thenceupwardly through a conduit 26 to join the exhaust gas inletted through25. The combined gas and air pass through conduit 29 into a separator30. This separator is of known centrifugal type and functions to removefrom the air stream fine particles which are introduced either throughthe conduit 25 or the stack 28. After passing through the centrifugalseparator the mixture of gas and air moves through a conduit 31 andenters a heat exchanger 32 which leads, through a passage 33, to acritical flow venturi shown generally at 34. The heat exchanger is of akind shown diagrammatically in FIG. 1, and constitutes an element whichwas hitherto essential to air sampling systems. As explained later inthis description, my invention also contemplates embodiments which omitthe heat exchanger. This is particularly important where exhaustemissions from a turbine engine are to be sampled, since the heatexchanger would be excessive in size.

The improved apparatus of FIG. 2 also includes a centrifugal blower 35which is coupled, by a sleeve and clamp assembly shown at 36, to theoutlet or expansion side of the venturi device 34.

The system of FIG. 2 is shown diagrammatically in FIG. 3, which latterfigure bears reference numerals corresponding to those appearing in FIG.2. The centrifugal separator 30 is not illustrated in FIG. 3, but itshould be understood that such a separator is preferably employed. Ithas the advantage of producing an intimate mixture of particulatematter, exhaust gas and air. As is the case with the positivedisplacement pump system of FIG. I, probes 18a and 18b are,respectively, disposed to sample the diluent air and the mix- The lengthof the venturi along its axis, measured from the inlet end at 37 to theoutlet 38, which latter flares outwardly from the expansion portion 39,is equal to 14.25 inches. The throat of the venturi is defined by thenarrow annular area represented in the drawing at 40 and having adimension parallel to the axis of the venturi and equal to 0.100 inch.The inlet portion of the venturi can be considered as being measuredfrom the reference line 40a to the plane of the inlet orifice 37 and isequal to 2.528 inches. The flare angle of this inlet portion is 30 withrespect to the axis of the venturi.

The expansionportion of the device extends from the reference line 40bto a lefthand line of reference shown at 41, a distance equal to 9.062inches and at a flare angle of 6. From the plane of termination of theture of exhaust and diluent air prior to delivery of the samples to airbags, not shown. The blower 35 of the system of FIG. 2 also appears inFIG. 3, and it is believed that operation of the apparatus of thisdiagrammatic figure will now be understood without further description.It should, however, be recognized that a system in accordance with FIGS.2 and 3 would also include the filters, valving and gas collectioninstrumentalities shown in the apparatus of FIG. 1, as well as thenecessary switching apparatus. The revolution counters of FIG. 1 are, ofcourse not required in a system of the kind diagrammatically representedin FIG. 3, since the system of the latter figure, as well as the otherembodiments of this invention, employ a venturi device rather than arotating positive displacement pump.

Turning now to FIG. 4 there is seen an enlarged cross sectionalillustration of the venturi device 34. This device is specificallydesigned to achieve a proper flow rate, in this case 300 CFM, whenoperating under conditions of critical flow. The venturi is the resultof mathematical analysis and development tests which require notreatment herein, since the nature thereof will be understood by thoseskilled in the art, it only being necessary to specify the desired rate(300 CFM at critical flow).

expansion portion of the venturi, i.e. reference line 41, the deviceflares at an angle of 279/minutes, with respect to the axisandterminates in the exit orifice located in the plane shown at 38 in FIG.4.

The venturi device is fabricated of suitable corrosion resistant steel,and has a wall thickness of 0.25 inch. The tubing 33 which extends intoadjacency with the inlet of the venturi terminates in a flanged portion42 which confronts a similar portion 43, formed on the venturi inlet. Acircular clamp 44 serves to secure the two flanges in confrontingrelation. At the opposite end of the venturi device the flared outlet isprovided with a circumferential flange 45 which is shaped and disposedto cooperate with a sleeve 46 forming a part of the housing ofcentrifugal blower 35. As mentioned earlier in this description, asleeve of resilient material surrounds the portions 45 and 46 and issecured by clamping structure 36. i

As will be understood from the foregoing description, the venturi 34serves as a means for metering the flow of gaseous mixture and maintainsthe flow rate of said mixture within very close limits. Since themixture of exhaust gas and diluent air flows through the venturithroughout a test, and because the pump means coupled to the expansionside of the venturi restriction is of capacity sufficient to insure thatthe restriction operates at conditions of critical gas flow throughoutany test period, the combined venturi and pump operate, inherently, as aconstant volume flow control device.

FIG. 5 illustrates a modified embodiment of the invention in which twosystems of the kind illustrated in FIG. 3 are operated in parallel, withcommon manifolding 47 adapted to receive the outflow of two venturidevices 34a and 34b. A single blower 35a, or other pressure reducingdevice, is coupled to the common manifold 47, with provision for finalexhausting of the output at 48. Such a multiple system, which mayinclude more than two venturi devices in parallel, is adapted to receivegaseous input from several internal combustion engines simultaneously,as is clear from the presence of exhaust pipe connections 25a and 25b.The probes 18c and 18d would, of course, be coupled with gas analyzerequipment (not shown).

In constant volume gas sampling, using embodiments of the kind describedthus far, the inlet gas temperature to the critical flow venturi must besubstantially constant. For this reason a heat exchanger, which includesmeans for heating or cooling the gases, has been provided andillustrated in the preceding embodiments.

An alternate approach, which provides a constant percentage sample flowrate without the necessity of a heat exchanger, is illustrated in FIG.6. in common with the systems already described, the mixture of exhaustgas and diluent air is fed to the inlet side of a venturi 34 which iscoupled to an excess capacity centrifugal pump 35. Gas samples arederived through a probe comprising a small tube 19' which, in thisembodiment, is provided with a venturi restriction as appears at R. Asmall blower B is provided. This blower is of sufficient capacity tomaintain critical flow within the venturi R. it can readily bedemonstrated that the quantity of flow taking place through the smallventuri R, as a percentage of the primary stream flow through venturi34', is constant and equal to the ratio of the areas of the twoventuris. Since the flow through each venturi is a function of thetemperature of the gas mixture, and since the same temperature prevailsat each venturi, no temperature compensation is necessary and the ratioof the areas is solely determinative of the respective flows. As will beunderstood, samples of gas derived from the venturi R and the blower Bwould be fed to collection equipment and suitable analyzing apparatus.Apparatus of this kind is particularly useful in the testing ofemissions from gas turbine engines. The term internal combustion, asused in the appended claims, should be understood as including suchengines within its scope.

I claim:

1. In apparatus for sampling the gaseous emissions of internalcombustion engines, for collection and analysis, a source of diluent airadapted for connection with a conduit carrying the exhaust of an engine,to produce a mixture of emissions and diluent air, means for derivingsamples of said mixture for analysis, and means for regulating the flowof said mixture to maintain the flow rate substantially constant, saidlast means comprising: a venturi restriction having an inlet side and anexpansion side, said inlet side being coupled in the apparatus toreceive said mixture; and pump means coupled to the expansion side ofsaid venturi restriction and effective to draw the mixture therethrough,said pump means being of capacity sufficient to insure that the venturirestriction operates at conditions of critical gas flow throughout testperiods.

2. Apparatus in accordance with claim 1, and in which said means forderiving samples includes hollow probe means of restricted cross-sectiondisposed in the stream of flowing mixture up-stream of said venturirestriction, and receptacle means coupled to receive the samples derivedthrough said probe means.

3. Apparatus in accordance with claim 2, and further including secondhollow probe means of restricted cross-section disposed in the stream ofdiluent air at a location to derive, for analysis, samples of such airat a region upstream of the point of connection of said conduit carryingthe engine exhaust.

4. Apparatus in accordance with claim 1, and in which said pump means isof the centrifugal type.

5. Apparatus in accordance with claim 1, and further characterized bythe inclusion of: means defining a plurality of flow paths disposed inparallel, each including its own source of diluent air and each adaptedfor connection to a separate engine; a plurality of venturi restrictionseach individual to a correspondin one of said flow paths; and a singlepump coupl in the system to draw the mixture flowing in each flow paththrough the venturi restriction included in that path.

6. Apparatus in accordance with claim 5, and in which said means for.deriving samples includes a plurality of probes of restrictedcross-section, each probe being disposed to sample the mixture flowingin a corresponding one of said flow paths.

7. Apparatus in accordance with claim 1, and in which said means forderiving samples is disposed in the region of said venturi restrictionand comprises: a conduit having a part disposed to receive a smallportion only of said mixture; a part defining in said conduit anauxiliary venturi restriction; and means insuring that said auxiliaryventuri restriction operates at conditions of critical gas flow duringsampling.

8. Apparatus in accordance with claim 7, and further characterized inthat the ratio of the minimum crosssectional areas of the two venturirestrictions is solely determinative of the quantity of flow througheach.

9. In apparatus for sampling the gaseous emissions of internalcombustion engines, for collection and analysis, a source of diluent airadapted for connection with a conduit carrying the exhaust of an engine,to produce a mixture of emissions and diluent air, means for derivingsamples of said mixture for analysis, and means for regulating the flowof said mixture to maintain the flow rate substantially constant, saidlast means comprising: a venturi restriction coupled in the apparatus toreceive said mixture; and pump means coupled to said venturi restrictionand effective to cause flow of the mixture therethrough, said pump meansbeing of a capacity sufficient to insure that the venturi restrictionoperates at conditions of critical gas flow throughout test periods.

10. In apparatus for sampling the gaseous emissions of internalcombustion engines for analysis, means providing an inlet source ofdiluent air, means coupling said source with a conduit carrying theexhaust of an engine, to produce a mixture of emissions and diluent air,conduit means through which said mixture passes and including heatexchanger means effective to maintain the mixture at a substantiallyconstant temperature throughout a test period, means coupled to saidconduit means and effective to derive samples of said mixture afterpassage through said heat exchanger means and to collect said samplesfor analysis, and means for regulating the flow of said mixture throughsaid apparatus to maintain the flow rate substantially constant, saidlast means comprising: means defining a venturi restriction having aninlet side and an expansion side, said inlet side being coupled to saidconduit means to receive the mixture flowing therethrough; and acentrifugal pump coupled to the expansion side of said venturirestriction and effective to draw the mixture therethrough, saidcentrifugal pump being of capacity sufficient to insure that the venturirestriction operates at conditions of critical gas flow throughout testperiods.

2. Apparatus in accordance with claim 1, and in which said means forderiving samples includes hollow probe means of restricted cross-sectiondisposed in the stream of flowing mixture up-stream of said venturirestriction, and receptacle means coupled to receive thE samples derivedthrough said probe means.
 3. Apparatus in accordance with claim 2, andfurther including second hollow probe means of restricted cross-sectiondisposed in the stream of diluent air at a location to derive, foranalysis, samples of such air at a region upstream of the point ofconnection of said conduit carrying the engine exhaust.
 4. Apparatus inaccordance with claim 1, and in which said pump means is of thecentrifugal type.
 5. Apparatus in accordance with claim 1, and furthercharacterized by the inclusion of: means defining a plurality of flowpaths disposed in parallel, each including its own source of diluent airand each adapted for connection to a separate engine; a plurality ofventuri restrictions each individual to a corresponding one of said flowpaths; and a single pump coupled in the system to draw the mixtureflowing in each flow path through the venturi restriction included inthat path.
 6. Apparatus in accordance with claim 5, and in which saidmeans for deriving samples includes a plurality of probes of restrictedcross-section, each probe being disposed to sample the mixture flowingin a corresponding one of said flow paths.
 7. Apparatus in accordancewith claim 1, and in which said means for deriving samples is disposedin the region of said venturi restriction and comprises: a conduithaving a part disposed to receive a small portion only of said mixture;a part defining in said conduit an auxiliary venturi restriction; andmeans insuring that said auxiliary venturi restriction operates atconditions of critical gas flow during sampling.
 8. Apparatus inaccordance with claim 7, and further characterized in that the ratio ofthe minimum cross-sectional areas of the two venturi restrictions issolely determinative of the quantity of flow through each.
 9. Inapparatus for sampling the gaseous emissions of internal combustionengines, for collection and analysis, a source of diluent air adaptedfor connection with a conduit carrying the exhaust of an engine, toproduce a mixture of emissions and diluent air, means for derivingsamples of said mixture for analysis, and means for regulating the flowof said mixture to maintain the flow rate substantially constant, saidlast means comprising: a venturi restriction coupled in the apparatus toreceive said mixture; and pump means coupled to said venturi restrictionand effective to cause flow of the mixture therethrough, said pump meansbeing of a capacity sufficient to insure that the venturi restrictionoperates at conditions of critical gas flow throughout test periods. 10.In apparatus for sampling the gaseous emissions of internal combustionengines for analysis, means providing an inlet source of diluent air,means coupling said source with a conduit carrying the exhaust of anengine, to produce a mixture of emissions and diluent air, conduit meansthrough which said mixture passes and including heat exchanger meanseffective to maintain the mixture at a substantially constanttemperature throughout a test period, means coupled to said conduitmeans and effective to derive samples of said mixture after passagethrough said heat exchanger means and to collect said samples foranalysis, and means for regulating the flow of said mixture through saidapparatus to maintain the flow rate substantially constant, said lastmeans comprising: means defining a venturi restriction having an inletside and an expansion side, said inlet side being coupled to saidconduit means to receive the mixture flowing therethrough; and acentrifugal pump coupled to the expansion side of said venturirestriction and effective to draw the mixture therethrough, saidcentrifugal pump being of capacity sufficient to insure that the venturirestriction operates at conditions of critical gas flow throughout testperiods.